A known linear solenoid linearly drives a movable core through use of a magnetic field that is generated upon energization of a coil of a stator. For example, JP2011-222799A (US2011/0248805A1) discloses a linear solenoid, which includes a coil and a bobbin. An outer peripheral part of the coil and an outer peripheral part of the bobbin are resin molded. The coil and the bobbin are held between two yokes together with the first stationary core and the second stationary core. The bobbin includes a winding portion, a first flange portion, a second flange portion and a projection. The winding portion is configured into a tubular form, and the coil is wound around the winding portion. The first flange portion is placed at one end part of the winding portion and radially outwardly extends from the one end part of the winding portion. The second flange portion is placed at the other end part of the winding portion, which is axially opposite from the one end part of the winding portion. The second flange portion radially outwardly extends from the other end part of the winding portion. The projection axially projects from the other end part of the winding portion toward a corresponding one of the yokes. An O-ring is placed between the one of the yokes and the second flange portion of the bobbin, and another O-ring is placed between the other one of the yokes and the first flange portion of the bobbin. In JP2011-222799A (US2011/0248805A1), the linear solenoid is used as a drive device of a hydraulic pressure change valve of a valve timing control apparatus of an internal combustion engine.
It is conceivable to form a fusing protrusion at the bobbin and to provide a housing that is resin molded together with the yoke, the coil and the bobbin. In such a case, the resin material of the housing is joined to and fused with the fusing protrusion of the bobbin. In this way, it is possible to eliminate the O-ring and to limit the intrusion of the oil, which is present in the inside of the linear solenoid, to the coil through the boundary surface between the housing and the bobbin. When the intrusion of the oil to the coil is limited, it is possible to limit conduction of the oil from the coil to an electronic control device through terminals connected to the coil.
However, heat of the coil, which is generated at the time of energizing the coil, tends to be kept in the inside of the molded member, so that release of the heat to the outside is not effectively carried out. When the heat is accumulated in the inside of the molded member, a resistance (electric resistance) of the coil is increased. At that time, even though the same voltage is applied to the coil, the current, which flows through the coil, becomes smaller in comparison to the previous state, which is before the occurrence of the change in the resistance of the coil. Therefore, the magnetic field, which is generated around the coil, is reduced, and thereby the magnetic attractive force, which attracts the mover, becomes smaller. In addition, when the heat is accumulated in the inside of the molded member, the molded member made of the resin material may be thermally degraded.
Furthermore, the pressure, which is exerted to the projection of the bobbin due to the flow of the resin material at the time of molding the housing, causes radially inward flexion of the projection. When the projection of the bobbin is radially inwardly flexed, a gap is formed between the bobbin and the yoke, and the molten resin material flows into this gap defined between the bobbin and the yoke to cause generation of burrs. Also, when the projection of the bobbin is radially inwardly flexed, a crack may be formed in the bobbin. The burrs may possibly be broken and may fall into sliding pats) of the linear solenoid and/or the sliding part(s) of the spool of the hydraulic pressure change valve. Also, the burrs may possibly intrude into the inside of the engine. When the crack is formed in the bobbin, the oil, which is present in the inside of the linear solenoid, may possibly be conducted from the crack to the electronic control device through the coil and the terminals to cause a damage of the electronic control device.